Liquified petroleum gas (LPG) such as butane and propane are particularly good fuels because of the clean combustion and the relatively high heat content for the weight. These fuels can easily be transported and stored in relatively inexpensive tanks which are also referred to as bottles. The fuel is stored as a liquid but its boiling point is lower than the typical ambient temperature. Therefore, there is a continual production of vapor under pressure. This pressurized vapor can easily and conveniently be burned to produce heat. Further, the vapor pressure at typical ambient temperatures is not so great as to require an extremely strong and expensive storage tank. As a result, the liquified fuel can be stored and transported in relatively inexpensive containers.
However, such fuels in the liquid state have a substantial coefficient of expansion. This expansion can produce extremely high pressures which can be much greater than the vapor pressure of the gas. It is therefore important that the storage tanks not be completely filled with fluid, since expansion of the fluid could rupture the tank. To prevent this problem the tank is not completely filled. There is left at the top of the tank a void termed a "head space". This space is made sufficiently large to absorb the maximum possible thermal expansion of the fluid.
A number of practices have been used in the past to insure the proper filling of propane and butane tanks. One method is for the operator to weigh the tank while it is being filled and turn off the service valve when the appropriate weight for the tank is reached. This requires that the operator know the weight and volume of the tank in advance and that he carefully monitor the filling to stop the input of flow at the appropriate time. Another method makes use of an overflow tube. This is a tube that extends into the tank down to the desired fluid level. When the fluid in the tank reaches this level, the fluid will be forced out through this tube into the air where it evaporates and produces a white cloud of vapor which indicates that the desired fluid level has been reached. The operator must then immediately shut off the service valve to prevent any further fluid from entering the tank. Both of these techniques require careful attention on the part of the operator to prevent the tanks from being excessively filled into the head space.
As a result of potential operator errors in the filling of LPG tanks, it has been recognized that there is a need for an automatic shut off valve to insure that the proper level of fluid is stored in the tank. One such valve is shown in U.S. Pat. No. 4,313,459 to Mylander. However, the automatic shutoff valves developed to date have not been able to meet all of the operational and regulatory requirements to make possible widespread use of such devices.
In response to the critical filling requirements for LPG tanks and the potential for very widespread use of such tanks by consumers, there have been developed regulations and performance standards which must be met by manufacturers and users of LPG. These standards include the minimum size of the head space as well as maximum leakage rates for an automatic shut off valve. In view of the consumer demands for LPG usage and the standards which have been set for such equipment, there exists a need for a stop fill valve which can reliably and effectively serve as an automatic shut off under a wide range of conditions while at the same time meeting the required standards, including maximum leakage rates.